A heart attack does not always have classic symptoms, such as pain in the chest, shortness of breath and cold sweats, researchers said.

It can occur without symptoms which is called a silent heart attack (when the blood flow to the heart muscle is severely reduced or cut off completely), they said. "The outcome of a silent heart attack is as bad as a heart attack that is recognised while it is happening," said Elsayed Z Soliman from Wake Forest Baptist Medical Centre in the US.

"And because patients do not know they have had a silent heart attack, they may not receive the treatment they need to prevent another one," said Soliman.

Researchers analysed the records of 9,498 middle-age adults already enrolled in the Atherosclerosis Risk in Communities (ARIC), a study analysing the causes and outcomes of atherosclerosis - hardening of the arteries.

They examined heart attack differences between blacks and whites as well as men and women. Over an average of nine years after the start of the study, 317 participants had silent heart attacks while 386 had heart attacks with clinical symptoms, researchers said.

They continued to follow participants for more than two decades to track deaths from heart attack and other diseases. Researchers found that silent heart attacks made up 45 per cent of all heart attacks and increased the chances of dying from heart disease by three times.

They also found that silent heart attacks increased the chances of dying from all causes by 34 per cent and were more common in men but more likely to cause death in women.

"Women with a silent heart attack appear to fare worse than men. Our study also suggests that blacks may fare worse than whites, but the number of blacks may have been too small to say that with certainty," said Soliman.

Symptoms of silent heart attacks appear so mild that they are barely noticed, if at all. They are detected later, usually when patients undergo an electrocardiogram, to check their heart's electrical activity, researchers said.

Soliman said that silent heart attacks, once discovered, should be treated as aggressively as heart attacks with symptoms.

"The modifiable risk factors are the same for both kinds of heart attacks. Doctors need to help patients who have had a silent heart attack quit smoking, reduce their weight, control cholesterol and blood pressure and get more exercise," said Soliman.